8,683 research outputs found
Time-delayed models of gene regulatory networks
We discuss different mathematical models of gene regulatory networks as relevant to the onset and development of cancer. After discussion of alternativemodelling approaches, we use a paradigmatic two-gene network to focus on the role played by time delays in the dynamics of gene regulatory networks. We contrast the dynamics of the reduced model arising in the limit of fast mRNA dynamics with that of the full model. The review concludes with the discussion of some open problems
Detecting Early Galaxies Through Their 21-cm Signature
New observations over the next few years of the emission of distant objects
will help unfold the chapter in cosmic history around the era of the first
galaxies. These observations will use the neutral hydrogen emission or
absorption at a wavelength of 21-cm as a detector of the hydrogen abundance. We
predict the signature on the 21-cm signal of the early generations of galaxies.
We calculate the 21-cm power spectrum including two physical effects that were
neglected in previous calculations. The first is the redistribution of the UV
photons from the first galaxies due to their scattering off of the neutral
hydrogen, which results in an enhancement of the 21-cm signal. The second is
the presence of an ionized hydrogen bubble near each source, which produces a
cutoff at observable scales. We show that the resulting clear signature in the
21-cm power spectrum can be used to detect and study the population of galaxies
that formed just 200 million years after the Big Bang.Comment: 5 pages, 3 figures, submitted to MNRAS Let
On Generating Gravity Waves with Matter and Electromagnetic Waves
If a homogeneous plane light-like shell collides head-on with a homogeneous
plane electromagnetic shock wave having a step-function profile then no
backscattered gravitational waves are produced. We demonstrate, by explicit
calculation, that if the matter is accompanied by a homogeneous plane
electromagnetic shock wave with a step-function profile then backscattered
gravitational waves appear after the collision.Comment: Latex file, 15 pages, accepted for publication in Physical Review
Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions
Recent models which describe our world as a brane embedded in a higher
dimensional space introduce new geometrical degrees of freedom: the shape
and/or size of the extra dimensions, and the position of the brane. These modes
can be coherently excited by symmetry breaking in the early universe even on
``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational
radiation. Two sources are described: relativistic turbulence caused by a
first-order transition of a radion potential, and Kibble excitation of
Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral
properties are estimated and the prospects for observation by LISA are
discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the
3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds
with energy peaked at observed frequencies in the LISA band, between
and Hz. The background is detectable above instrument and
astrophysical foregrounds if initial metric perturbations are excited to a
fractional amplitude of or more, a likely outcome for the
Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys.
Rev. Let
A Characterisation of Strong Wave Tails in Curved Space-Times
A characterisation of when wave tails are strong is proposed. The existence
of a curvature induced tail (i.e. a Green's function term whose support
includes the interior of the light-cone) is commonly understood to cause
backscattering of the field governed by the relevant wave equation. Strong
tails are characterised as those for which the purely radiative part of the
field is backscattered. With this definition, it is shown that electromagnetic
waves in asymptotically flat space-times and fields governed by tail-free
propagation have weak tails, but minimally coupled scalar fields in a
cosmological scenario have strong tails.Comment: 17 pages, Revtex, to appear in Classical and Quantum Gravit
Researching teachersâ time use: Complexity, challenges and a possible way forward
In the context of global concerns about teacher workload and the relationship between workload and attrition, understanding the nature, quantity and intensity of teachersâ work is an essential first step in formulating robust solutions to this significant problem. Understanding teachersâ work, however, is a complex undertaking, and prior attempts have largely been focused on the quantity rather than the intensity or quality of work required and undertaken. This article reports on a pilot study of the Teacher Time Use app, a bespoke tool developed by the research team to âget insideâ teachersâ subjective experience of time through a focus on both workload and intensity. Our analysis shows that the app provides a simple, non-demanding way for teachers to record their work in a timely and efficient way. It also highlights the capacity of this approach to understand both the range and quantum of tasks that comprise teachersâ work and consequently the nature and subjective experience of work intensification. We argue the need for a more nuanced empirical understanding of the layering and multi-tasking of teachersâ work that characterises work intensity, and suggest that the Teachersâ Time Use app provides an effective means for recording and representing the complex dimensions of teachersâ work and time use
A comparative framework: how broadly applicable is a 'rigorous' critical junctures framework?
The paper tests Hogan and Doyle's (2007, 2008) framework for examining critical junctures. This framework sought to incorporate the concept of ideational change in understanding critical junctures. Until its development, frameworks utilized in identifying critical junctures were subjective, seeking only to identify crisis, and subsequent policy changes, arguing that one invariably led to the other, as both occurred around the same time. Hogan and Doyle (2007, 2008) hypothesized ideational change as an intermediating variable in their framework, determining if, and when, a crisis leads to radical policy change. Here we test this framework on cases similar to, but different from, those employed in developing the exemplar. This will enable us determine whether the framework's relegation of ideational change to a condition of crisis holds, or, if ideational change has more importance than is ascribed to it by this framework. This will also enable us determined if the framework itself is robust, and fit for the purposes it was designed to perform â identifying the nature of policy change
Halo Cores and Phase Space Densities: Observational Constraints on Dark Matter Physics and Structure Formation
We explore observed dynamical trends in a wide range of dark matter dominated
systems (about seven orders of magnitude in mass) to constrain hypothetical
dark matter candidates and scenarios of structure formation. First, we argue
that neither generic warm dark matter (collisionless or collisional) nor
self-interacting dark matter can be responsible for the observed cores on all
scales. Both scenarios predict smaller cores for higher mass systems, in
conflict with observations; some cores must instead have a dynamical origin.
Second, we show that the core phase space densities of dwarf spheroidals,
rotating dwarf and low surface brightness galaxies, and clusters of galaxies
decrease with increasing velocity dispersion like Q ~ sigma^-3 ~ M^-1, as
predicted by a simple scaling argument based on merging equilibrium systems,
over a range of about eight orders of magnitude in Q. We discuss the processes
which set the overall normalization of the observed phase density hierarchy. As
an aside, we note that the observed phase-space scaling behavior and density
profiles of dark matter halos both resemble stellar components in elliptical
galaxies, likely reflecting a similar collisionless, hierarchical origin. Thus,
dark matter halos may suffer from the same systematic departures from homology
as seen in ellipticals, possibly explaining the shallower density profiles
observed in low mass halos. Finally, we use the maximum observed phase space
density in dwarf spheroidal galaxies to fix a minimum mass for relativistically
decoupled warm dark matter candidates of roughly 700 eV for thermal fermions,
and 300 eV for degenerate fermions.Comment: Submitted to the Astrophysical Journal, LaTeX, 26 pages including 4
pages of figure
Optical integral field spectroscopy of intermediate redshift infrared bright galaxies
The extreme infrared (IR) luminosity of local luminous and ultra-luminous IR
galaxies (U/LIRGs; 11 12,
respectively) is mainly powered by star-formation processes triggered by
mergers or interactions. While U/LIRGs are rare locally, at z > 1, they become
more common, they dominate the star-formation rate (SFR) density, and a
fraction of them are found to be normal disk galaxies. Therefore, there must be
an evolution of the mechanism triggering these intense starbursts with
redshift. To investigate this evolution, we present new optical SWIFT integral
field spectroscopic H{\alpha}+[NII] observations of a sample of 9
intermediate-z (0.2 < z < 0.4) U/LIRG systems selected from Herschel 250{\mu}m
observations. The main results are the following: (a) the ratios between the
velocity dispersion and the rotation curve amplitude indicate that 10-25% (1-2
out of 8) might be compatible with being isolated disks while the remaining
objects are interacting/merging systems; (b) the ratio between un-obscured and
obscured SFR traced by H{\alpha} and LIR, respectively, is similar in both
local and these intermediate-z U/LIRGs; and (c) the ratio between 250{\mu}m and
the total IR luminosities of these intermediate-z U/LIRGs is higher than that
of local U/LIRGs with the same LIR . This indicates a reduced dust temperature
in these intermediate-z U/LIRGs. This, together with their already measured
enhanced molecular gas content, suggests that the interstellar medium
conditions are different in our sample of intermediate-z galaxies when compared
to local U/LIRGs.Comment: Accepted for publication in MNRA
Monte Carlo Simulation of Lyman Alpha Scattering and Application to Damped Lyman Alpha Systems
A Monte Carlo code to solve the transfer of Lyman alpha (Lya) photons is
developed, which can predict the Lya image and two-dimensional Lya spectra of a
hydrogen cloud with any given geometry, Lya emissivity, neutral hydrogen
density distribution, and bulk velocity field. We apply the code to several
simple cases of a uniform cloud to show how the Lya image and emitted line
spectrum are affected by the column density, internal velocity gradients, and
emissivity distribution. We then apply the code to two models for damped Lya
absorption systems: a spherical, static, isothermal cloud, and a flattened,
axially symmetric, rotating cloud. If the emission is due to fluorescence of
the external background radiation, the Lya image should have a core
corresponding to the region where hydrogen is self-shielded. The emission line
profile has the characteristic double peak with a deep central trough. We show
how rotation of the cloud causes the two peaks to shift in wavelength as the
slit is perpendicular to the rotation axis, and how the relative amplitude of
the two peaks is changed. In reality, damped Lya systems are likely to have a
clumpy gas distribution with turbulent velocity fields, which should smooth the
line emission profile, but should still leave the rotation signature of the
wavelength shift across the system.Comment: 19 pages, 17 eps figures. One panel is added in Fig.1 to show the
recoil effect. Revisions are made in response to the referee's comments.
Accepted for publication in Ap
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